Refactor CI

[mjp41]

This restructures the CI to make it clearer what is different between each build by using reusable workflows.

[SchrodingerZhu]

If it is easier and helpful, you may want to directly absorb #808 into this.

[Copilot]

Pull request overview

This PR refactors the CI configuration to centralize common build logic into reusable workflows, making the matrix of platforms and build variants clearer and easier to maintain.

Changes:

• Introduces reusable-cmake-build.yml to share CMake configure/build/test logic across Ubuntu, macOS, and Windows jobs, including optional self-host and size checks.
• Introduces reusable-vm-build.yml to unify FreeBSD and NetBSD VM-based builds, with parameterized VM type, version, and dependency commands.
• Restructures .github/workflows/main.yml to consume the reusable workflows for the various build jobs (Ubuntu, macOS, BSDs, Windows, QEMU cross, GWP-ASan, etc.) and narrows Dependabot’s Cargo updates to the snmalloc-rs subdirectory.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
.github/workflows/reusable-vm-build.yml	New reusable workflow encapsulating FreeBSD/NetBSD/OpenBSD VM setup, CMake configuration, Ninja build and ctest invocation, parameterized by VM type and version.
.github/workflows/reusable-cmake-build.yml	New reusable workflow that standardizes dependency installation, CMake configuration, build, testing, size checks, and optional self-hosting across OSes and generators.
.github/workflows/main.yml	Refactored main CI workflow to use the new reusable workflows for Ubuntu, macOS, BSD, Windows, QEMU cross-builds, fuzzing, GWP-ASan, and the final gate, and extended build/test matrix variants (sanitizers, self-vendored STL, etc.).
.github/dependabot.yml	Updated Dependabot configuration so Cargo updates target the snmalloc-rs subdirectory instead of the repository root and removed the gitsubmodule ecosystem entry.

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[Copilot]

In the qemu-crossbuild job, matrix.arch.extra-packages is only defined for the riscv64 entry, but the Install cross-compile toolchain and QEMU step always runs sudo apt install ${{matrix.arch.extra-packages}}. For architectures like armhf and arm64 where extra-packages is unset, this expands to sudo apt install with no package names, which will cause the step (and therefore those jobs) to fail. Consider either folding extra-packages into the first apt install command or guarding the second apt invocation with a condition so it only runs when extra-packages is non-empty.

Suggested change

	sudo apt install ${{matrix.arch.extra-packages}}
	if [ -n "${{ matrix.arch.extra-packages }}" ]; then
	sudo apt install ${{ matrix.arch.extra-packages }}
	fi

[Copilot]

For the NetBSD VM build, the previous workflow explicitly set both the C and C++ compilers to the GCC 10 toolchain, but the new reusable workflow only passes -DCMAKE_CXX_COMPILER=/usr/pkg/gcc10/bin/g++ via cmake-flags. This means the C compiler will now fall back to the system default, which is inconsistent with the earlier configuration and can make debugging compiler / ABI issues harder; if the intent is still to build both C and C++ with GCC 10, consider also setting -DCMAKE_C_COMPILER=/usr/pkg/gcc10/bin/gcc in cmake-flags.

Suggested change

	cmake-flags: -DCMAKE_CXX_COMPILER=/usr/pkg/gcc10/bin/g++
	cmake-flags: -DCMAKE_C_COMPILER=/usr/pkg/gcc10/bin/gcc -DCMAKE_CXX_COMPILER=/usr/pkg/gcc10/bin/g++